Novel 5-aryl-[1,2,4]triazoloquinazoline Fluorophores: Synthesis, Comparative Studies of the Optical Properties and ICT-Based Sensing Application

A synthetic approach to novel tricyclic fluorophores has been developed using 2-aryl-[1,2,4]triazolo[1,5-c]quinazoline-5(6H)-ones as key precursors. Additionally, π-extended analogues were obtained via Suzuki–Miyaura cross-coupling of 5-(4-bromophenyl)-[1,2,4]triazoloquinazoline with arylboronic acids. All compounds were fully characterized. The structure of target fluorophores was further confirmed by X-ray single crystal diffraction. Photophysical investigations revealed bright blue fluores-cence in toluene (ΦF > 95%) for all 5-aminoaryl-substituted [1,2,4]triazolo[1,5-c]quinazoline derivatives. Incorporation of a 9,9‘-spirobisfluorene moiety led to blue-shifted absorption and emission, as well as significant reduced quantum yield. Introduction of a 1,4-phenylene spacer had little effect on the absorption, but caused a notable batho-chromic shift in the emission – up to 56 nm in toluene and 142 nm in acetonitrile. [4,3-c]-Annelated analogue showed hypsochromic absorption shifts and moderate quan-tum yield (ΦF < 34%) with unusual solvent-dependent quenching. Selected compounds exhibited pronounced solvatochromism, with emission maxima shifting by over 100 nm between non-polar and polar solvents. Furthermore, distinct acid-induced fluorescence changes were observed upon protonation, for selected compounds, indicating potential applicability as dual-mode (polarity and pH) molecular sensors. Density functional theo-ry (DFT) calculations supported the experimental findings, revealing intramolecular charge transfer (ICT) characteristics for selected molecules.